1. Field of the Invention
The present invention relates to a biomedical magnetic field measuring apparatus. More particularly, this invention is concerned with a biomedical magnetic field measuring apparatus having a field detection unit in which a fragile magnetic field generated in a living body is sensed by a pickup coil and then led to a superconducting ring referred to as a superconducting quantum interference device (SQUID).
2. Description of the Related Art
In recent years, a SQUID fluxmeter serving as a biomedical magnetic field measuring apparatus in which superconducting quantum interference devices (SQUIDs) are used to measure magnetic fluxes in a living body has been put to practical use. The SQUID fluxmeter falls into a radiofrequency (rf)-SQUID type and direct current (dc)-SQUID type. The dc-SQUID type is used generally these days because sensitivity is excellent and few noises occur.
In the case of the dc-SQUID type, when a dc bias current of a level disabling retention of a superconducting state is supplied to a junction of a superconducting ring, if a magnetic field originating from a living body is detected using a pickup coil and led to the superconducting ring, a periodically-varying voltage to the detected magnetic field is induced in the junction. From this viewpoint, when the voltage in the junction varies, a magnetic flux canceling the variation is applied from a feedback coil to the superconducting ring so that a voltage proportional to a current flowing through the feedback coil can be read externally. A circuit thus configured is referred to as a flux locked loop (FLL) circuit. Owing to this circuit, an output proportional to a magnetic field detected by a pickup coil can be obtained.
A typical configuration of a pickup coil array employed in such a SQUID fluxmeter is shown in FIG. 1, and various forms of windings of pickup coils are shown in FIGS. 2A to 2C. A pickup coil array 100 shown in FIG. 1 has a plurality of first-order differential type pickup coils 101 arranged along a curved coil arrangement surface so that the axial-directions of the coils 101 will be substantially perpendicular to the surface of the head of a patient. This configuration is currently the mainstream of a SQUID fluxmeter. A magnetometer type pickup coil (the order of a differential is 0) shown in FIG. 2A, a first-order differential type pickup coil shown in FIG. 2B, and a second-order differential type pickup coil shown in FIG. 2C are used as the pickup coil 101. Above all, the second-order differential type pickup coil has the better ability to remove a magnetic field originating from an external noise than the first-order differential type coil, and is therefore employed in measuring a magnetic field in the environment with a larger external noise. The first-order differential type pickup coil has the poorer ability to remove an external noise and is therefore desirably used for measurement in a magnetic shielding room. The magnetometer type pickup coil does not have the ability to remove an external noise and is therefore unused in general. As shown in FIGS. 2B and 2C, a distance in axial direction between two coil loop planes is referred to as a base line. A conventional (differential type) pickup coil array is characterized by the fact that a plurality of pickup coils having the same base line are arranged on a curved coil arrangement surface. Incidentally, a third-order or higher-order differential type pickup coil in which consideration is taken into the durability to an external noise has been conceived.
It has been revealed that if information on current sources in a living body is inferred from the results of a measurement performed using a biomedical magnetic field measuring apparatus including a pickup coil array having the foregoing structure, there is the fear of bringing about drawbacks described below.
(1) When a distribution of current sources distributed in a living body three-dimensionally is inferred using a solution like a linear least squares method, a distribution of current sources that are distributed at shallower positions than they actually are is liable to be inferred.
(2) A plurality of current dipoles are imagined in a living body, if the positions, sizes, and orientations of the dipoles are inferred through non-linear optimization, small magnetic fields generated by dipoles located at deeper positions are hidden behind large magnetic fields generated by dipoles located at shallower positions. It is therefore hard to detect the dipoles at deeper positions accurately.
In an effort to solve this problem, a proposal has been made for a method in which a plurality of, for example, first-order differential type pickup coils 102 are, as shown in FIG. 3, arranged in steps in its axial direction, and information on current sources in a living body is inferred from magnetic field strengths detected by these pickup coils 102.
However, a magnetic field measuring apparatus including a pickup coil array having the structure described in conjunction with FIG. 3 has a problem that since a magnetic field in a living body detected by a pickup coil located away from the living body is small, the signal-to-noise ratio of a pickup coil located far away becomes inferior to that of a pickup coil located nearby. As a whole, a contribution is hardly made to improvement of accuracy in inference.